One month on from US's deadliest mudslide

One month on from the deadly mudslide which engulfed the small town of Oso in Washington state, US President Obama visited the area, offering support to the devastated community.The President took an aerial tour of the site where at least 41 people were killed last month when a rain-saturated hillside along the Stillaguamish River collapsed, sweeping a torrent of mud, rock and trees thought the settlement. Roads and homes were engulfed in a dense mass, covering an area 460m long, 1,300m wide, and some 20m deep in some places.According to the local Bellevue Fire Department, the debris contains toxic sludge, human waste and toxic chemicals. There are reports that some rescue workers have come down with dysentery.Oso, which lies 100km northeast of Seattle had been made vulnerable to the risk of a mudslide by record-breaking rainfall during the previous three weeks. In fact, March turned out to be the wettest since records began in 1891, with 240mm. This beat the previous record of 213mm set back in 1950.The death toll makes the Oso mudslide the deadliest in US history. Yet the region experiences frequent mudslides. The geological feature, of which the recent activity was a part, is known as the Hazel Landslide. This has a history of movement which dates back to the 1930s.Much of the area is made up of loosely bound sands and gravels, left behind by the retreat of glaciers at the end of the last Ice Age. Some of these deposits were left hanging 200m above the current valley floor.When these deposits are lacking in clay particles, which has a binding effect, they are susceptible to water infiltration. This has a lubricating effect, creating a zone of structural weakness.Although the exceptional rainfall is being seen as the main cause of last month’s catastrophic event, questions are also being asked about logging which has taken place above the town. Investigations are being carried out to determine whether log clearance impinged upon the flow of groundwater into the mudslide zone.As rescue efforts continue to find the bodies of two people still unaccounted for, President Obama said he wanted to show support for “families who are searching for loved ones [and] families who have lost everything.”In the coming days search efforts are likely to be hampered by heavy rain which is expected to continue across the Pacific Northwest.